Explanation

Rutherford's 1911 gold-foil experiment fired alpha particles at a thin sheet of gold. Most particles passed straight through, but a small fraction bounced back at large angles. Only a tiny, dense, positively charged mass at the atom's centre could repel heavy, positively charged alpha particles so strongly. The experiment therefore proved the existence of a positively charged nucleus (C). It says nothing about electrons being the deflecting mass (A/B) or about isotopes (D).